EP3302985B1

EP3302985B1 - Imaging device

Info

Publication number: EP3302985B1
Application number: EP16734029.8A
Authority: EP
Inventors: Nir RUBIN BEN HAIM; Michael Nagler; Benzion Landa; Tamar Kashti; Ofer Aknin; Ronen YOGEV; Itai Tzur
Original assignee: Landa Labs 2012 Ltd
Current assignee: Landa Labs 2012 Ltd
Priority date: 2015-05-27
Filing date: 2016-05-27
Publication date: 2020-12-16
Anticipated expiration: 2036-05-27
Also published as: US10061200B2; AU2016268512A1; EP3302985A1; KR20180013952A; US20170097573A1; MX2017015246A; EP3302986A1; CN107667317B; CA2986370C; RU2017138078A; IL255928A; JP2021092796A; CA2986370A1; ES2842209T3; KR20180013951A; IL255931B; JP2018518392A; WO2016189511A9; CN107667316A; WO2016189510A9

Description

FIELD

The present disclosure relates to an imaging device for projecting a plurality of individually controllable laser beams onto a surface that is movable relative to the imaging device. The imaging device will be described herein mainly by reference to its application in digital printing systems but its use is not limited to this application.

BACKGROUND

US 7,002,613 describes a digital printing system to which the imaging device of the present disclosure is applicable. In particular, in Figure 8 of the latter patent specification, there is shown an imaging device designated 84 that is believed to represent the closest prior art to the present disclosure. The imaging device serves to project a plurality of individually controllable laser beams onto a surface, herein termed an imaging surface, to generate an energy image onto that surface. The laser image can be used for a variety of purposes, just a few examples being to produce a two dimensional printed image on a substrate, as taught for instance in US 7,002,613 , in 3D printing and in etching of an image onto any surface.
For high throughput applications, such as commercial printing or 3D lithography, the number of pixels to be imaged every second is very high, demanding parallelism in the imaging device. The laser imaging device of the present disclosure is intended for applications that require energy beams of high power. One cannot therefore merely scan the imaging surface with a single laser beam, so as to expose the pixels sequentially. Instead, the imaging device is required to have a separate laser emitting element for each pixel (picture element) of the image area of the imaging surface.
To achieve acceptable print quality, it is important to have as high a pixel density as possible. A high resolution image, for example one having 1200 dpi (dots per inch), requires a density of laser emitting elements that is not achievable if the laser emitting elements all lie in a straight line, due to the amount of overlap necessary between the laser sources to achieve a uniform printing quality. Aside from the fact that it is not physically possible to achieve such a high packing density, adjacent elements would interfere thermally with one another.
Semiconductor chips are known that emit beams of laser light in an array of M rows and N columns. In US 7,002,613 the rows and columns are exactly perpendicular to each other but the chips are mounted askew, in the manner shown in Figure 1 of the latter patent, so that each row can fill in the missing pixels of the preceding row(s). In this way, such an array can achieve a high resolution image but only over the width of the chip and such chips cannot simply be mounted side by side if one is to achieve a printed image without stripes along its length, because the chips cannot have laser emitting elements positioned sufficiently close to their lateral edges.
US 7,002,613 avoids this problem by arranging such chips in two rows, in the manner shown in Figure 8 of the latter patent. The chips in each row are staggered relative to the chips in the other row of the pair so that each chip in one row scans the gap left unscanned by the two adjacent chips in the other row.
Even though it is expected that the rows of chips will be mounted on a support under clean laboratory conditions using a microscope to achieve their correct alignment, it is not possible to guarantee that the relative alignment of the chips in the two rows will be accurate within the resolution of the printed image. Any misalignment will result in the image having stripes or other undesired defects.
US 2010/080594 and US 2008/181667 describe systems in which the light from arrays of LED's (rather than laser sources) is projected onto an image surface and teach how steps may be taken to compensate for any misalignment between the arrays. In each case, the images produced by adjacent arrays are overlapped and selected LED's from one or other of the two arrays are activated to maintain image continuity at the boundary between the two arrays. In the case of US 2010/080594 this overlap is shown clearly in Figure 14 and in US 2009/181667 it is evident, for example, from Figures 9A and 9B.

SUMMARY

In the present disclosure, there is proposed an imaging device for projecting individually controllable laser beams onto an imaging surface that is movable relative thereto in a reference X-direction, the device including a plurality of semiconductor chips each of which comprises a plurality of individually controllable laser beam emitting elements arranged in a two dimensional main array of M rows and N columns, the elements in each row having a uniform spacing (A_r) and the elements in each column having a uniform spacing (a_c), wherein the chips are mounted on a support in such a manner that each pair of chips that are adjacent one another in a reference Y-direction, transverse to the X-direction, are offset from one another in the X-direction, and, when activated continuously, the emitted laser beams of the two chips of said pair trace on the imaging surface 2•M•N parallel lines that extend in the X-direction and are substantially uniformly spaced in the Y-direction, the laser beams of each chip tracing a set of M•N lines that do not overlap the set of lines of the other chip, wherein in addition to the M rows and N columns of elements of the main array, each chip comprises at least one additional column on each side of the main array, each containing at least one selectively operable laser emitting element capable of compensating for any misalignment in the Y-direction in the relative positioning of the adjacent chips on the support by tracing at least one additional line that lies between the two sets of M•N lines.
Assuming that the M rows and N columns of laser emitting elements of the main array do not include any elements that are normally redundant, the spacing between adjacent lines in the set will be equal to A_r/M, namely the spacing of the adjacent elements in each row divided by the number of rows. Furthermore, because in the present disclosure there is no overlap between the two sets of M•N lines traced by any two adjacent chips, the total number of lines traced by the two chips will be equal to 2•M•N, namely twice the product of the number of rows and the number of columns in each chip, if the chips have equal numbers of rows and columns.
In the present proposal, in addition to these evenly spaced lines produced by the main arrays, additional laser emitting elements are provided at both ends of each array intended only to compensate for chip misalignment. If adjacent chips are correctly aligned, the elements of the additional columns will be redundant and will not be energized. However, if a gap should remain between the lines traced by adjacent chips, the additional elements can introduce additional lines to fill that gap at a position approximating to the uniform spacing of the lines traced by the main arrays. It should be noted that, in contrast to the proposals in US 2010/080594 and US 2008/181667 , the lines traced by the additional elements do not fall between (i.e. are not interlaced with) the lines of either set of M•N lines traced by the main arrays and only fall within any gap between the two sets of M•N lines.
In the event of an overlap between the two sets of M•N lines traced by the elements of adjacent chips, some of the elements of the main arrays can be switched off and if necessary replaced by an element of the additional columns to maintain the appearance of a raster with uniformly spaced lines.
On the other hand, if there is a gap between the two M•N lines traced by the elements of adjacent chips, the additional columns can be activated to maintain the appearance of a raster with uniformly spaced lines.
One of the additional elements may be activated on its own if its position coincides with a line that would render the raster uniform. Alternatively, if the elements have a symmetrical energy profile, resembling for example a Gaussian or a sinusoidal distribution, it is possible, by activating both elements to irradiate adjacent spots on the imaging surface and separately adjusting the power of each element, to produce a single raster line at an adjustable distance from the raster lines of the main elements of the two chips. It should be noted that this effect is thermally dynamic and additive provided that the adjacent spots are irradiated within a finite time of each other. In other words, the imaging surface should not have time to dissipate the energy of the first laser pulse in the interval between the two laser pulses.
Conveniently, the lines traced by elements in the same additional column are evenly spaced from one another, the spacing between the lines traced by the element of the additional column being substantially equal to the quotient of the spacing of the lines traced by the elements of main array and the number of elements in the additional column.
While it would be possible to use chips in which the rows and columns of the main arrays of laser emitting elements are perpendicular to one another, as taught in US 7,002,613 , this requires the chips to be placed at an angle relative to the Y-direction. In some embodiments of the present disclosure the elements in each row of each chip lie on a line parallel to the Y-direction and the elements in each column of each chip lie on a straight line inclined at an angle to the X-direction. In other words, instead of the outline of the array being square, the array is shaped as a parallelogram. This arrangement, which may be considered slightly wasteful as far as chip area is concerned, can be advantageous in terms of assembling procedure.
It is convenient for the chips to be arranged in at least one pair of rows on the support, with corresponding laser emitting elements of all the chips in each of the two rows lying in line with one another in the Y-direction. By "corresponding elements" it is meant that the individual laser emitting elements should occupy the same row and column positions within their respective chips. It is advantageous for corresponding elements in any group of three chips in the pair of rows that are adjacent one another in the X and Y-directions to lie at the apices of congruent equilateral triangles. This simplifies the construction of a lens system that is needed to focus the laser beams onto the imaging surface.
It has been found particularly advantageous for all the laser beams emitted by one chip to be focused on the imaging surface by a common single lens, or a common set of lenses arranged in series, having a magnification of ±1. In other words, the image of the array of laser elements on the imaging surface (i.e. an array of dots) should have the same size as the array on the chip, though it may be inverted. In such a case even if there is a slight misalignment of the lenses, such as GRIN rod (Gradient-Index) lenses, in the XY plane perpendicular to the optical axis of the lens, the position of the illuminated laser spot on the imaging surface will remain unchanged, as it only depends on the position of the laser emitting element on the laser array chip. The former elements can be positioned with very high accuracy on every laser array chip using standard semiconductor manufacturing techniques.
While the lens system may comprise a single GRIN rod associated with each chip, it may alternatively comprise a plurality of GRIN rods arranged in series with one another and forming a folded light path. In the latter case, a prism common to all the chips may serve to direct the laser beams from one GRIN rod element to the next in each series.
In such a folded light path configuration, it is desirable for the prism to be made of a glass having a higher refractive index than the GRIN rods.
It is convenient for each chip to have in the main array an equal number of rows and columns of laser beam emitting elements (i.e., M = N), as this minimizes the size of the lens array.
Within each chip, the separation between the laser elements should be sufficiently great to avoid thermal interference between adjacent laser emitting elements.
The support for the chip arrays may be fluid cooled to help dissipate the heat that may be generated by the chips.
Furthermore, the support may be a rigid metallic or ceramic structure and it may be formed of, or coated with, an electrically insulating surface bearing film conductors to supply electrical signals and power to the chips.
The chips in some embodiments are vertical cavity surface emitting laser (VCSEL) chip arrays.
In some embodiments, the intensity of the laser beam emitted by each element may be adjustable either continuously (in an analogue manner) or in discrete steps (digitally). In one embodiment, the chips may include D/A converters so as to receive digital control signals. In this way, the laser beam intensity may be adjusted in 4, 8, 16, 32 or up to 4096 discrete steps.
In a further aspect of the present disclosure, there is provided a method of projecting individually controllable laser beams onto an imaging surface that is movable relative to an imaging device utilising the imaging device of the present disclosure.
In some embodiments, at least one pair of laser elements, selected either both from the same array or one from each of two adjacent arrays, are controlled in such a manner that their energies are combined on the imaging surface to increase the temperature of the imaging surface above a predetermined threshold at a point intermediate the centers of the images of the two laser elements on the imaging surface, without raising the temperature of the imaging surface at at least one of the centers of the images of the two laser elements above the latter threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the imaging device are described herein with reference to the accompanying drawings. The description, together with the figures, makes apparent to a person having ordinary skill in the art how the teachings of the disclosure may be practiced, by way of non-limiting examples. The figures are for the purpose of illustrative discussion and no attempt is made to show structural details of an embodiment in more detail than is necessary for a fundamental understanding of the disclosure. For the sake of clarity and simplicity, some objects depicted in the figures are not to scale.
In the Figures:

Figure 1 is a schematic diagram of a digital printing system utilizing an imaging device according to an embodiment of the present disclosure;
Figure 2 shows part of an imaging device comprising a set of VCSEL chips mounted on a support;
Figure 3 is a schematic representation of the laser emitting elements of two VCSEL chips and the lines that they can trace on a relatively moving imaging surface;
Figure 4 is a schematic representation that demonstrates in one pair of rows the alignment between the VCSEL chips and the GRIN rods used as lenses to focus the emitted laser beams onto the imaging surface;
Figures 5A shows prior art proposals for correction of chip misalignment;
Figure 5B shows the manner in which the present disclosure compensates for chip misalignment;
Figure 6 shows the energy profiles produced by the laser elements at the ends of two adjacent arrays, to illustrate how a single line can be traced using two laterally positioned laser elements, there being shown for each array three elements of the main array and one of the additional elements;
Figure 7A is a similar energy diagram to Figure 6 to show how the energies of two adjacent laser elements of the main array can be combined on the imaging surface to produce an additional dot that does not fall on the center line of either of the laser elements;
Figure 7B shows the dot pattern on the imaging surface produced by activating four laser elements of the main array in the manner shown in Figure 7A;
Figure 8A shows how the dot pattern of Figure 7B assists in anti-aliasing;
Figure 8B shows for comparison with Figure 8A the jagged edge that normally occurs when printing an oblique line; and
Figure 9 shows an alternative lens system to that shown in Figure 1 that has a folded light path to permit more compact packaging in a printing system.

DETAILED DESCRIPTION

Overall description of an exemplary printing system

Figure 1 shows a drum 10 having an outer surface 12 that serves as an imaging surface. As the drum rotates clockwise, as represented by an arrow, it passes beneath a coating station 14 where it acquires a monolayer coating of fine particles. After exiting the coating station 14, the imaging surface 12 passes beneath an imaging device 15 of the present disclosure where selected regions of the imaging surface 12 are exposed to laser radiation which renders the particle coating on the selected regions of the surface 12 tacky. Next, the surface passes through an impression station 19 where a substrate 20 is compressed between the drum 10 and an impression cylinder 22. This causes the selected regions of the coating on the imaging surface 12 that have been rendered tacky by exposure to laser radiation by the imaging device 15 in the correspondingly termed imaging station to transfer from the imaging surface 12 to the substrate 20.
The term "tacky" as used herein is intended to mean that the irradiated particle coating is not necessarily tacky to the touch but only that it is softened sufficiently to be able to adhere to the surface of a substrate when pressed against it in the impression station 19.
The regions on the imaging surface 12 corresponding to the selected tacky areas transferred to the substrate consequently become exposed, being depleted by the transfer of particles. The imaging surface can then complete its cycle by returning to the coating station 14 where a fresh monolayer particle coating is applied only to the exposed regions from which the previously applied particles were transferred to the substrate 20 in the impression station 19.
In the present proposal, because the particles adhere to the imaging surface more strongly than they do to one another, the applied coating of particles, also interchangeably termed a particle layer or coating, is substantially a monolayer. While some overlap may occur between particles, the layer may be only one particle deep over a major proportion of the area of the surface and most, if not all, of the particles will have at least some direct contact with the imaging surface. Thus, the average thickness of the monolayer can be approximated by the average thickness of the individual particles forming it or, in some regions where particles overlap, by a low multiple of the dimension of the constituting particles, depending on the type and extent of the overlap. A monolayer may therefore have a maximum thickness (T) corresponding to up to about three times a thinnest dimension characteristic to the particles involved (e.g., the thickness of the particles for flake shaped ones or essentially the particle diameter for spherical ones).
The formation of a substantial monolayer mosaic of particles occurs for the same reason that an adhesive tape, when used to pick up a powder from a surface, will only pick up one layer of powder particles. When the adhesive tape is still fresh, the powder will stick to the adhesive until it covers the entire tape surface. However, once the adhesive has been covered with powder, the tape cannot be used to pick up any more powder because the powder particles will not stick strongly to one another and can simply be brushed off or blown away from the tape. Similarly, the monolayer herein is formed from the particles in sufficient contact with the imaging surface and is therefore typically a single particle thick. Contact is said to be sufficient when it allows the particle to remain attached to the imaging surface at the exit of the coating device.
Advantageously, a monolayer of particles facilitates the targeted delivery of radiation as emitted by the laser elements of an imaging device according to present teachings. This may ease the control of the imaging device and process, as the selectively irradiated particles reside on a single defined layer. When considered for use in a printing system, an imaging device targeting a monolayer can preferably focus the laser radiation to form upon transfer to a substrate a dot of approximately even thickness and/or relatively defined contour.
Another advantage of having a monolayer is that it can provide for good thermal coupling between the particles (e.g., polymers including pigments or dyes, for printing applications) and the imaging surface on which the particles are coated. As shall be described below, the imaging surface may be a heat absorbing substrate or made of a suitably heat absorbing material, thus easing the transfer of energy from the imaging surface to the polymer particle(s) which render them tacky. It should be mentioned that because of the very small thickness of the particles, most of the laser energy can pass through them without being absorbed. Instead of heating the particles directly, the laser radiation tends instead to heat the imaging surface and the particles are heated indirectly.

The coating station

Reverting to the coating station 14, it may comprise a plurality of spray heads 1401 that are aligned with each other along the axis of the drum 10 and only one is therefore seen in the section of Figure 1. The sprays 1402 of the spray heads are confined within a bell housing 1403, of which the lower rim 1404 is shaped to conform closely to the imaging surface leaving only a narrow gap between the bell housing 1403 and the drum 10. The spray heads 1401 are connected to a common supply rail 1405 which supplies to the spray heads 1401 a pressurized fluid carrier (gaseous or liquid) having suspended within it the fine particles to be used in coating the imaging surface 12. In the present disclosure, the term "suspended in" and its variations is to be understood as "carried by" and like terms, not referring to any particular type of mixture of materials of same or different phase in any particular fluid, which may be optionally maintained at a desired controlled temperature.
If needed the suspended particles may be regularly or constantly mixed, in particular before their supply to the spray head(s). The particles may for instance be circulated in the coating apparatus within a flow rate range of 0.1 to 10 liter/minute, or in the range of 0.3 to 3 liter/min. The fluid and the surplus particles from the sprays heads 1401, which are confined within a plenum 1406 formed by the inner space of the housing 1403, are extracted through an outlet pipe 1407, which is connected to a suitable suction source represented by an arrow, and can be recycled back to the spray heads 1401. Though herein referred to as spray heads, any other type of nozzle or orifice along the common supply pipe or conduit allowing applying the fluid suspended particles are encompassed.
As an alternative to directly spraying of the particles and their carrier onto the imaging surface, it is possible for them to be sprayed onto an applicator, such as a rotating brush or sponge that then applies the particles to the imaging surface. For comprehensive coverage of the imaging surface, several such applicators may be contained in the coating station, which may have additional sponges for drying the imaging surface before it leaves the coating station.
It is important to be able to achieve an effective seal between the housing 1403 and the imaging surface 12, in order to prevent the spray fluid and the fine particles from escaping through the narrow gap that must essentially remain between the housing 1403 and the imaging surface 12 of the drum 10. Different ways of achieving such a seal are shown schematically in the drawing.
The simplest form of seal is a wiper blade 1408. Such a seal makes physical contact with the imaging surface and could score the applied coating if used on the exit side of the housing 1403, that is to say the side downstream of the spray heads 1401. For this reason, if such a seal is used, it is preferred for it to be located only upstream of the spray heads 1401 and/or at the axial ends of the housing 1403. The terms "upstream" and "downstream" as used herein are referenced to points on the imaging surface 12 as it cycles through the different stations.
Figure 1 also shows how egress of the fluid within which the particles are suspended from the sealing gap between the housing 1403 and the drum 10 can be prevented without a member contacting the imaging surface 12. A gallery 1409 extending around the entire circumference of the housing 1403 is connected by a set of fine passages 1410 extending around the entire rim of the housing 1403 to establish fluid communication between the gallery 1409 and the sealing gap.
In a first embodiment, the gallery 1409 is connected to a suction source of a surplus extraction system, which may be the same suction source as is connected to the outlet 1407 or a different one. In this case, the gallery serves to extract fluid passing through the gap before it exits the housing 1403. The low pressure also sucks off the drum 10 any particles that are not in direct contact with the imaging surface 12 and, if the sprayed fluid is a liquid, it also sucks off surplus liquid to at least partially dry the coating before it leaves the coating station 14. Surplus liquid can alternatively and additionally be removed by mean of a liquid extracting roller (e.g., having a liquid absorbing surface) positioned on the exit side of the coating apparatus. Any such means of drying the particle coating (e.g., a blower, a heater, a liquid extractor etc.), if present, can be internal to the coating device 14 (i.e., within plenum 1406 of housing 1403), or can alternatively be positioned downstream of the coating station, as long as it remains upstream of a station where the coating needs to be substantially dry. The drying element, if present, is advantageously compatible with the particle layer, and for instance does not negatively affect the particles and/or the integrity of the layer formed therefrom.
In an alternative embodiment, the gallery 1409 is connected to a source of gas at a pressure higher than the pressure in the plenum 1406. Depending on the rate of fluid supply to the plenum through the spray heads 1401 and the rate of extraction through the outlet 1407, the plenum 1406 may be at a pressure either above or below the ambient atmospheric pressure.
If the plenum is at sub-atmospheric pressure, then is suffices for the gallery 1409 to be at ambient pressure, or indeed no gallery need be present. In this case, because the pressure within the sealing gap will exceed the pressure in the plenum 1406, gas flow through the gap will be towards the interior of the housing with no risk of fluid egress.
If the plenum is at above atmospheric pressure, then the gallery 1409 may be connected to a pressurized gas supply, preferably air. In this case, air will be forced into the sealing gap under pressure through the passages 1410 and will split into two streams. One stream will flow towards the plenum 1406 and will prevent egress of the fluid within which the particles are suspended. That stream will also dislodge and/or entrain particles not in direct contact with the imaging surface and assist in drying the coating if the carrier fluid is a liquid. The second stream will escape from the coating station without presenting a problem as it is only clean air without any suspended particles. The second gas stream may also assist in further drying of the particle coating on the imaging surface 12 before it leaves the coating station 14. If desired, the gas stream can be heated to facilitate such drying.
In an alternative embodiment, the afore-mentioned gallery 1409 does not extend around the entire circumference of the housing, so as to seal the plenum 1406 on all sides. It can be a "partial" gallery or a combination of one or more air knives (with negative or positive flow) positioned either downstream or upstream of the spray heads in parallel to the axis of the drum and/or on the lateral edges of the spray heads in a direction perpendicular to the axis of the drum. A "partial" gallery on the exit side may, in some embodiments, serve as gas blower (e.g., cold or hot air) additionally or alternatively facilitating the drying of the particles, in which case the passages 1410 may be adapted to provide sufficient flow rate.
In one embodiment, and independently of the type of fluid carrying the suspended particles being applied to the imaging surface, there is included on the exit side of the coating apparatus 14, and typically at an external downstream location, a heater allowing the temperature of the particle layer and the imaging surface to be raised before it reaches the imaging station 160. The temperature of the particles and the imaging surface may in this way be raised from ambient temperature to above 30°C, or 40°C or even 50°C, so as to reduce the amount of laser energy that is needed to render the particles tacky. However, the heating should not itself render the particles tacky and should not raise their temperature to above 80°C or possibly to above 70°C. Such heating of the particles and imaging surface may be further facilitated by using a fluid carrier at desired temperature.
In some embodiments, there can be included on the entry side of the coating apparatus 14, and typically at an external upstream location, a cooler allowing lowering the temperature of the imaging surface before the particle layer is being replenished in the previously exposed regions. It is believed that an imaging surface at a temperature of less than 40°C, or less than 30°C, or even less than 20°C, but typically above 0°C, or even above 10°C, can reduce the temperature of the particles neighboring the exposed regions so that by the time the imaging surface is being replenished, the so cooled particles may have no or reduced "residual tackiness", that is to say a partial softening insufficient for a subsequent step (e.g., transfer to a printing substrate). The cooled coating behaves in the same manner as the particles freshly deposited on the exposed regions of the imaging surface. In this manner, only particles selectively targeted by any laser element of a chip of an imaging device as herein disclosed would become sufficiently tacky for a subsequent transfer step. Such cooling of the particles and imaging surface may be further facilitated by using a fluid carrier at desired temperature.
It is possible to provide both a cooler on the entry side of the coating apparatus 14 and a heater on the exit side, each cooler and heater operating as above described. Additionally, the drum 10 can be temperature controlled by suitable cooling / heating means internal to the drum, such temperature controlling means being operated, if present, in a manner to allow the outer surface of the imaging surface to be maintained at any desired temperature.

The imaging surface

The imaging surface 12 in some embodiments is a hydrophobic surface, made typically of an elastomer that can be tailored to have properties as herein disclosed, generally prepared from a silicone-based material. The hydrophobicity assists in the separation of the particles from the imaging surface after they have been made tacky by exposure to radiation so at to allow the particles to transfer cleanly to the substrate without splitting.
A surface is said to be hydrophobic when the angle formed by the meniscus at the liquid/air/solid interface, also termed wetting angle or contact angle, exceeds 90°, the reference liquid being typically distilled water. Under such conditions, which are conventionally measured with a goniometer or a drop shape analyzer and can be assessed at a given temperature and pressure of relevance to the operational conditions of the coating process, the water tends to bead and does not wet, hence does not adhere, to the surface.
The imaging surface 12 may have any Shore hardness suitable to provide a strong bond to the particles when they are applied to the surface in the coating station 14, the bond being stronger than the tendency of the particles to adhere to one another. The suitable hardness may depend on the thickness of the imaging surface and/or the particles intended to be bond. In some embodiments, a relatively high hardness between about 60 Shore A and about 80 Shore A is suitable for the imaging surface. In other embodiments, a medium-low hardness of less than 60, 50, 40, 30 or even 20 Shore A is satisfactory. In a particular embodiment, the imaging surface has a hardness of about 40 Shore A.
Advantageously, an imaging surface suitable for use with an imaging device herein disclosed can be flexible enough to be mounted on a drum, have sufficient abrasion resistance, be inert to the particles and/or fluids being employed, and/or be resistant to any operating condition of relevance (e.g., irradiation, pressure, heat, tension, and the like).
To be compatible with the radiation intermittently generated by the imaging station to expose desired selected areas, the imaging surface can, for instance, be relatively resistant and/or inert to the radiation, and/or able to absorb the radiation, and/or able to retain the heat generated by the radiation.
The imaging surface 12 in the drawing is the outer surface of a drum 10 but this is not essential as it may alternatively be the surface of an endless transfer member having the form of a belt guided over guide rollers and maintained under an appropriate tension at least while it is passing through the coating station.

The particles

The particles may be made of any material and have any shapes and/or dimensions suitable to provide for sufficient contact area with the imaging surface, at least over a time period the particle coating is desired. Advantageously the material of the particles can be rendered sufficiently tacky by the laser elements so as to selectively transfer.
The shape and composition of the particles will depend in practice on the intended use of the layer of particles, and in the context of a non-limiting example of a printing system, on the nature of the effect to be applied to the surface of the substrate 20. The particles may, for instance, comprise a thermoplastic polymer and optionally a coloring agent (e.g., a pigment or a dye) and have a near spherical shape. The particles may further include a softening facilitating agent (e.g., an IR absorbing dye) tuned to the wavelength emitted by the laser element, and preferably not affecting the desired color of the particle, having if necessary substantially no absorbance in the visible part of the spectrum. For printing of high quality, it is desirable for the particles to be as fine as possible to minimize the interstices between particles of the applied monolayer coating. The particle size is dependent upon the desired image resolution and for some applications a particle size (e.g., a diameter) of 10 µm (micrometers) may prove adequate. However, for improved image quality, it is preferred for the particle size to be a few micrometers and more preferably less than about 1 µm. In some embodiments, suitable particles can have an average diameter between 100 nm and 4 µm, in particular between 500 nm and 1.5 µm.
Thus particle selection and ideal size determination, will depend upon the intended use of the particles, the effect sought (e.g., visual effect in the case of printing), and the operating conditions of the relevant system in which a coating device and imaging device according to the present teachings is to be integrated. Optimization of the parameters may be done empirically, by routine experimentation, by one of ordinary skill in the art.
Depending on their composition and/or on the processes they undergo, the particles can be hydrophobic with different degrees, if any, of hydrophilicity. As the balance between the hydrophobic and hydrophilic nature of the particles may shift with time, the process is expected to remain efficient if the hydrophobic nature of the particles predominates. Additionally, the particles may be made of materials intrinsically hydrophilic, in which case they can be rendered hydrophobic by application of a suitable particle coating.
The particles can be carried by either a gaseous or a liquid fluid when they are applied onto the imaging surface or upon the intermediate applicator(s). When the particles are suspended in a liquid, in order both to reduce cost and minimize environmental pollution, it is desirable for the liquid to be aqueous. In such a case, it is desirable for the polymer or material used to form or coat the particles to be hydrophobic. Hydrophobic particles more readily separate from an aqueous carrier, facilitating their tendency to attach to and coat the imaging surface. Such preferential affinity of the particles towards the surface of the coating device, rather than towards their carrier and towards one another, is deemed particularly advantageous. Blowing a gas stream over the particle coating (which as mentioned can preferably be formed by hydrophobic particles on an hydrophobic imaging surface) will both serve to dislodge particles not in direct contact with the imaging surface and to dry the particle coating on the imaging surface.
The above description is not intended to provide a comprehensive explanation of the operation of the entire digital printing system. Many details that are important for a successful implementation of such a printing system are not relevant to the present disclosure. However, the above description of the printing system of Figure 1 is believed to be sufficient to enable the exemplary function that can be served by the imaging device of the present disclosure to be understood. It should, furthermore, be stressed that the imaging device is capable of being used for other purposes, for example selectively activating regions of an adhesive, etching a metal foil carried by the imaging surface or curing a polymer in a 3D printing system.

The imaging device

The imaging device 15 in Figure 1 is composed of a support 16 carrying an array of laser sources such as VCSEL (vertical cavity surface emitting laser) chips that emit laser beams and an array of corresponding lenses 18 that focus the laser beams on the imaging surface 12. Figures 2 to 4 provide more details of the chips and the manner in which they are mounted on the support and aligned with the lenses 18.
Figure 2 shows the support 16 on which are mounted a plurality of VCSEL chips 30 arranged in two rows in accurately predetermined positions relative to one another, as will be described in more detail by reference to Figures 3 and 4 .
The support 16 is a rigid at least partially hollow elongate body fitted with connectors 34 to allow a cooling fluid to flow through its internal cavity. The body of the support may be made of an electrically insulating material, such as a suitable ceramic, or it may be made of a metal and at least its surface 36 on which the chips 30 are mounted may be coated with an electrical insulator. This enables a circuit board made of thin film conductors (not shown in the drawing) to be formed on the surface 36. The chips 30 are soldered to contact pads on this circuit board and a connector 32 projecting from the lower edge of the support 16 allows control and power signals to be applied to the chips 30. The laser emitting elements 40 of each chip 30 are individually addressable and are spaced apart sufficiently widely not to interfere thermally with one another.
In some embodiments, the individually controllable laser elements of a chip can emit laser beams having variable energy that is preferably digitally controllable in discrete steps, allowing the laser intensity to be set at any of 4, 8, 16 .... up to 4096 levels. The lowermost level of energy is defined as 0, where the individual laser element is not activated, the uppermost level of energy can be defined as 1. Such distinct levels may be considered analogous in the field of printing to "grey levels", each level providing for a gradually distinct intensity (e.g., shade when considering a colored output). Taking for instance, a laser beam emitting element having 16 levels of activation, level 0 would result in lack of impression (e.g., leaving a substrate bare or white if originally so) and level 1 would result in transfer of a tacky film formed by a particle irradiated at maximum energy (e.g., forming a full black dot in the event the particles are so colored). In previous illustrative example, levels 1/16, 2/16, 3/16 and so on would correspond to increasingly stronger shades of grey, comprised between white (0) and black (1). Typically, the energy levels are evenly spaced.
In an alternative embodiment, the individually controllable laser elements of a chip can emit laser beams having variable energy that can be modulated in a continuous analogue manner.
Once a region of the imaging surface has reached a temperature at which the particles become tacky, any further increase in temperature will not have any effect on the transfer to the substrate. However, it should also be noted that as the intensity of the laser is increased the size of the dot that is rendered tacky also increases.
The energy profile of each dot resembles the plots shown in Figure 6 , that is to say that it is symmetrical with tapering sides. The exact profile is not important as the distribution may be Gaussian, sinusoidal or even an inverted V. In any such profile, as the peak intensity increases, the base widens and the area of intersection of the profile with a threshold at which the particle coating is rendered tacky also increases in diameter. A consequence of this energy distribution is that points of the imaging surface that are not in alignment with the centerline of any one laser emitting element will receive energy from adjacent elements. It is possible for two nearby elements to be energized to below the level needed to render coating particles on the centerline of the elements tacky, yet for the cumulative energy in the region of overlap between the two centerlines to rise above the level necessary to render the coating particles tacky. In this way, it is possible to create potential raster lines between the centerlines of the laser lines in addition to, or as an alternative to, the raster lines coinciding with the centerlines of the laser elements. This ability to combine the energies from adjacent elements is used to achieve different effect, as will be described below. These effects are dependent upon the ability of the imaging surface to combine energies received from different laser elements, even if there is a slight difference between the times of irradiation.
Figure 3 shows schematically, and to a much enlarged scale, the relative positioning of two laser emitting element arrays 130a and 130b of VCSEL chips 30 that are adjacent one another in the Y-direction but are located in different rows. Each of the chips has a regular array of M by N laser emitting elements 40, as previously described, that are represented by circular dots. In the example illustrated, M and N are equal, there being nine rows and nine columns. The spacing between the elements in a row, designated A_r, and the spacing between the elements in a column, designate a_c, are shown as being different from one another but they may be the same. The array is shown as being slightly skewed so that the columns and rows are not perpendicular to one another. Instead, the rows lie parallel to the Y-direction while the columns are at a slight angle to the X-direction. This enables lines, such as the lines 44, traced by the elements 40 on the imaging surface, if energized continuously, to be sufficiently close together to allow high resolution images to be printed. Figure 3 shows that the element at the end of each row traces a line that is a distance A_r/M away from the line traced by the corresponding element of each adjacent row, the separation between these lines being the image resolution I_r. Thus A_r and M are selected in dependence upon the desired image resolution, based on the equation A_r = M x I_r.
It should be mentioned that it is possible for the elements to lie in a square array where the columns are perpendicular to the rows. In this case, the chips would need to be mounted askew on their support and compensation would need to be applied to the timing of the control signals used to energize the individual elements.
As is clear from Figure 3 , and also Figure 5B which shows the traced lines to a larger scale, the positioning of the array 130b is such that the line traced by its bottom left element 40 should ideally also be spaced from the line traced by the top right element of the array 130a by a distance equal to A_r/M. Therefore when all the elements 40 of both arrays 130a and 130b are energized, they will trace 2•M•N lines that will all be evenly spaced apart by a distance A_r/M between adjacent lines, without any gaps.
If one wishes to provide compensation for defective elements, the array could include additional rows of laser emitting elements 40, but it is alternatively possible to compensate for a defective element by increasing the intensity of the laser beams generated by the laser emitting elements that trace the two adjacent parallel lines.
In addition to the M by N array of elements 40, each chip has two additional columns that are arranged one each side of the main array, each containing a respective further element 42. These further elements 42 are represented in Figure 3 by stars, to distinguish them from the main array elements 40. The additional laser elements on each side of each array, can be positioned at a distance of 1/3 the spacing between traced lines that are imaged by the lenses onto the imaging surface. Furthermore additional elements could be placed in the gap between two arrays that nominally spans a distance of A_r/M so that higher sensitivity is achieved in correcting the spacing errors between adjacent arrays.
As can be seen from Figure 3 and Figure 5B , when activated, these elements 42 trace two additional lines 46 between the two sets of evenly spaces parallel lines 44a and 44b traced by the elements 40 of the two arrays 130a and 130b, respectively.
One of the additional lines 46 is spaced by a distance A_r/3M from the last adjacent line 44a traced, for example, by the array 130a in Figure 3 and the other is spaced by a distance A_r/3M from the first adjacent line 44b traced, for example, by the array 130b. In the event of a misalignment between the two arrays 130a and 130b these elements 42 can be energized in addition to, or instead of some of, the elements 40 of the main arrays to compensate for any misalignment between the arrays 130a and 130b that tends to create a stripe in the printed image, be it a gap or a dark line resulting from an overlap. Figure 5A , which is similar to Figure 5B , shows the alternative approach proposed in the prior art to compensate for chip misalignment. In the prior art, each chip has an additional row of elements that produces traced lines that are interlaced with the traced lines of the adjacent chip, resulting in a very high degree of redundancy.
While the two additional elements 42 in the present proposal are shown in Figure 3 and Figure 5B as tracing two separate lines 46, the energies of these two elements can be combined on the imaging surface, as earlier described, to form a single line of which the position is controllable by appropriate setting of the energies emitted by each of the additional elements 42. This is shown in Figures 6 in which the energy profiles of the lines 44a and 44b are designated 94a and 94b, respectively and the energy profiles of the additional lines 46 are designated 96a and 96b. In Figure 6, neither of the profiles 96a and 96b (shown in dotted lines) has sufficient energy to render the coating particles tacky but at the centerline between the two arrays the cumulative energy, shown as a solid dark line 96, is sufficient to soften the particles coating and to create a trace line filling the gap between the trace lines 44a and 44b of the two main arrays.
While in Figure 6 the energy profiles of the two additional elements are matched, it is possible by varying the relative intensity of the two beams emitted by the additional laser sources to position the centerline of the combine energy at a different distance from the traces of the main arrays.
Figure 7A shows how the ability to create dots that do not fall on the centerlines of the energy profiles of the laser elements can be used to advantage to achieve anti-aliasing. Figure 7A shows the energy profiles of four adjacent elements of the main array. The first two profiles a and b are set at a desired level, say 8 (out of sixteen), corresponding to mid-grey. The energy profiles c and d, on the other hand are set to say 12 and 4, respectively. The resulting dot pattern produced on the imaging surface is shown in Figure 7B . This can be seen to comprise two regular sized dots A and B aligned with the line of symmetry of the profiles a and b in Figure 7A, a larger sized dot C aligned with the centerline of energy profile c , and a smaller dot D that lies somewhere between the centerlines of the profiles c and d .
The result of repeating such a dot pattern diagonally is shown in Figure 8A . When this image is compared with Figure 8B , where no anti-aliasing steps have been taken, it will be seen that the small dots in between regular raster line yield oblique edges that have reduced jaggedness and produce an image that is comparable with one achievable by a printing system having a greater image resolution.
The interaction of energies from nearby laser elements can also be used to compensate for missing elements in that the elements producing the two adjacent raster lines can be used to combined in the same manner as previously explained to fill in a gap between them.
For the arrays 130a and 130b in Figure 3 to function correctly as described above, their relative position in the Y-direction is critical. In order to simplify the construction of the lens system serving to focus the emitted laser beams on the imaging surface it is advantageous to adopt a configuration shown in Figure 4 which enables the two rows of lenses corresponding to a pair of chip rows to be self-aligning.
Figure 4 shows seven adjacent arrays 130 each shown lined up with a respective lens 18. Though arrays 130 can as afore-mentioned include additional laser elements 42, such are not shown on the present figure. Each lens 18 is constructed as a GRIN (Gradient-Index) rod, this being a known type of lens that is shaped as a cylinder having a radially graduated refractive index. In the case of the geometry shown in Figure 4 , corresponding elements of any three bi-directionally adjacent arrays 130 lie on the apices of an equilateral triangle, three such triangles designated 50 being shown in the drawing. It will be noted that all the triangles 50 are congruent. As a result, if the diameter of the GRIN rods is now selected to be equal to 2•N•A_r, which is the length of the sides of the equilateral triangles 50, or the distance between corresponding laser emitting elements of adjacent VCSEL chips 30 in the same row, then when stacked in their most compact configurations, the lenses 18 will automatically align correctly with their respective chip.
Though the lens 18 has been schematically illustrated in Figure 1 (side view) and Figure 4 (cross section view) as being an individual GRIN rod, in an alternative embodiment shown in Figure 9 the laser beams of each chip can be transmitted by a series of lenses. In the case of Figure 9 , the single GRIN rod 18 is replaced by two mutually inclined GRIN rods 18a and 18b and the light from one is directed to the other by a prism 87 of high refractive index glass, so that the light follows a folded path. Such a configuration enables coating stations in a colour printing system to be arranged closer to one another in a more compact configuration. Such a folded light path can adopt different configurations while fulfilling all the requirements of magnification and light transmission. To enable the light path to be split in this manner, the length of the GRIN rods is selected such that light is collimated on leaving the rods 18a and entering the rods 18b as shown by the light rays drawn in Figure 9 .
The radiation guided by GRIN rod 18a, the proximal end of which is arranged at a distance WD_o from the chip, may be captured by the corresponding GRIN rod 18b which can collect the collimated light emerging from rod 18a on the same light path and focus it at a distance WD_i from the distal end of the second GRIN rod 18b. When the two GRIN rods are made of the same material and the same radial gradient profile and WD_o = WD_i a magnification of M=+1 can be obtained.
Laser elements that are away from the longitudinal axis of the GRIN rod 18a will leave the distal end of the GRIN lens collimated but at an angle to the axis. In certain cases, it is necessary for the distance between the two rods 18a and 18b to be large, causing the off axis collimated beams exiting the first rod segment to miss partially or entirely the second segment. It is possible to take advantage of Snell's law and cause the beam exiting the first rod to travel through a glass with a high refractive index, thus causing the angle the collimated beam makes with the optical axis to decrease and enabling a larger separation between the rods before the collimated beams leaving the first rod miss the entrance to the second rod.
In the description and claims of the present disclosure, each of the verbs, "comprise" "include" and "have", and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements, steps or parts of the subject or subjects of the verb. These terms encompass the terms "consisting of' and "consisting essentially of'.
As used herein, the singular form "a", "an" and "the" include plural references and mean "at least one" or "one or more" unless the context clearly dictates otherwise.
Positional or motional terms such as "upper", "lower", "right", "left", "bottom", "below", "lowered", "low", "top", "above", "elevated", "high", "vertical", "horizontal", "backward", "forward", "upstream" and "downstream", as well as grammatical variations thereof, may be used herein for exemplary purposes only, to illustrate the relative positioning, placement or displacement of certain components, to indicate a first and a second component in present illustrations or to do both. Such terms do not necessarily indicate that, for example, a "bottom" component is below a "top" component, as such directions, components or both may be flipped, rotated, moved in space, placed in a diagonal orientation or position, placed horizontally or vertically, or similarly modified.
Unless otherwise stated, the use of the expression "and/or" between the last two members of a list of options for selection indicates that a selection of one or more of the listed options is appropriate and may be made.
In the disclosure, unless otherwise stated, adjectives such as "substantially" and "about" that modify a condition or relationship characteristic of a feature or features of an embodiment of the present technology, are to be understood to mean that the condition or characteristic is defined to within tolerances that are acceptable for operation of the embodiment for an application for which it is intended.

Claims

An imaging device for projecting individually controllable laser beams onto an imaging surface that is movable relative thereto in a reference X-direction, the device including a plurality of semiconductor chips (30) each of which comprises a plurality of individually controllable laser beam emitting elements (40) arranged in a two dimensional main array of M rows and N columns, the elements in each row having a uniform spacing A_r and the elements in each column having a uniform spacing a_c, wherein the chips are mounted on a support (16) in such a manner that each pair of chips that are adjacent one another in a reference Y-direction, transverse to the X-direction, are offset from one another in the X-direction, and, when activated continuously, the emitted laser beams of the two chips of said pair trace on the imaging surface 2•M•N parallel lines that extend in the X-direction and are substantially uniformly spaced in the Y-direction, the laser beams of each chip tracing a set of M x N lines that do not overlap the set of lines of the other chip, wherein in addition to the M rows and N columns of elements of the main array, each chip comprises at least one additional column on a side of the main array, each containing at least one selectively operable laser emitting element (42) capable of compensating for any misalignment in the Y-direction in the relative positioning of the adjacent chips on the support by tracing at least one additional line that lies between the two sets of M•N lines.
The imaging device as claimed in claim 1, wherein each additional column comprises two or more elements.
The imaging device as claimed in any one of the preceding claims, wherein the distance between the line traced by one of the additional laser emitting elements, and the adjacent line of the main array, is equal to the quotient of (i) the spacing of the lines traced by the elements of main array and (ii) a number greater by one than the number of elements in the additional columns.
The imaging device as claimed in any one of the preceding claims, wherein the elements in each row of each chip lie on a line parallel to the Y-direction and the elements in each column of each chip lie on a straight line inclined at an angle to the X-direction.
The imaging device as claimed in any one of the preceding claims, wherein the chips are arranged in pair of rows on the support, corresponding laser emitting elements of all the chips in each of the two rows lying in line with one another in the Y-direction.
The imaging device as claimed in claim 5, wherein the alignment of the chips within the two rows of the pair is such that corresponding elements in any group of three adjacent chips in the X and Y-directions lie at the apices of congruent equilateral triangles.
The imaging device as claimed in claim 6, wherein a respective lens is provided for each chip to focus the laser beams emitted by all the elements of the associated chip onto the imaging surface.
The imaging device as claimed in claim 7, wherein each lens is formed by a single GRIN rod.
The imaging device as claimed in claim 7, wherein each lens is formed by a series of two or more mutually inclined GRIN rods.
The imaging device as claimed in any one of claim 8 or 9, wherein the GRIN rods have a diameter equal to 2•N•A_r, being the distance between corresponding elements of adjacent chips in each row.
The imaging device as claimed in any one of the preceding claims, wherein the support is fluid cooled.
The imaging device as claimed in any one of the preceding claims, wherein the support is constructed of a rigid metallic or ceramic structure, the surface of the support is formed of, or coated with, an electrical insulator bearing film conductors to supply electrical signals and power to the chips.
The imaging device as claimed in any one of the preceding claims, wherein the chips are vertical cavity surface emitting laser (VCSEL) chip arrays.
The imaging device as claimed in any one of the preceding claims, wherein each individually controllable laser beam element can emit a laser beam having 4 levels of energy or more, or 8 levels of energy or more, or 16 levels of energy or more, or even 32 levels of energy or more.
A method of projecting individually controllable laser beams onto an imaging surface that is movable relative to an imaging device utilising the imaging device according to any one of the preceding claims.